The present invention relates to a shaped, Raney metal fixed-bed catalyst which contains at least one catalyst alloy made of a catalyst metal and an extractable alloying component, wherein the catalyst is activated in a surface layer with a thickness of 0.1 to 2.0 mm starting from the surface of the shaped catalyst by complete or partial extraction of the extractable alloying component and which optionally contains promoters.
In a further aspect, the present invention also relates to a process for preparing the catalyst by mixing a powder of the catalyst alloy and a high molecular weight polymer, shaping the mixture to form shaped articles and removing the polymer by thermal treatment and calcining the shaped articles at temperatures of less than 850xc2x0 C. Still further, the present invention also relates to use of the aforementioned catalyst for hydrogenation, dehydrogenation and hydrogenolysis reactions.
Activated metal catalysts are known in the field of chemical engineering as Raney catalysts. They are used, generally in powdered form, for a large number of hydrogenation reactions of organic compounds.
These powdered catalysts are prepared from an alloy of a catalytically active metal, which in the following description is also called a catalyst metal and a further alloying component which is soluble in alkalis. Nickel, cobalt, copper or iron are mainly used as catalyst metals. Aluminum is largely used as the alloying component which is soluble in alkalis, but other components may also be used, in particular zinc and silicon are also suitable.
This so-called Raney alloy is first finely milled in accordance with Raney""s method. Then the aluminum is removed entirely or partly by extracting with alkalis such as, for example, caustic soda solution.
This process activates the alloy powder. Due to extraction of the aluminum, the alloy powder has a high specific surface area, between 20 and 100 m2/g (BET), and is rich in adsorbed hydrogen. The activated catalyst powder is pyrophoric and is stored under water or organic solvents or embedded in high boiling organic compounds.
Powdered catalysts have the disadvantage that they can only be used in batch processes and have to be isolated after the catalytic reaction by filtering the reaction medium, a costly process. Therefore a number of processes for preparing shaped articles which lead to activated metal fixed-bed catalysts after extraction of the aluminum have been disclosed.
U.S. Pat. No. 4,826,799 describes the preparation of activated Raney metal fixed-bed catalysts by mixing a powder of the alloy of catalyst metal and aluminum with an organic polymer and optionally a shaping aid, shaping this mixture by extrusion or compression to give the desired shaped articles and calcining the shaped articles in air at temperatures above 850xc2x0 C. This leads to a pore structure in the shaped article, due to combustion of the added organic material, and to the formation of xcex1-aluminum oxide which acts as a ceramic binder between the alloy particles and provides the shaped articles with the desired mechanical stability. Then the shaped articles are activated by extracting the remaining aluminum which has not been oxidized during calcination.
A critical feature of this process is the formation of xcex1-aluminum oxide between the alloy particles as a ceramic binder. xcex1-aluminum oxide, in contrast to xcex3-aluminum oxide and aluminum, is not soluble in alkalis and is therefore not dissolved out during activation of the shaped article with caustic soda solution.
Catalysts prepared in accordance with U.S. Pat. No. 4,826,799 have serious disadvantages. In order to form xcex1-aluminum oxide, the shaped articles must be calcined at a temperature above 850xc2x0 C. In fact, below 850xc2x0 C. no xcex1-aluminum oxide but only xcex3-aluminum oxide, which is soluble in alkalis, is formed. The xcex1-aluminum oxide used as binder is catalytically inactive and thus reduces the catalyst activity. During calcination a well-sealed or less well-sealed sealed layer of this inactive material which is insoluble in alkalis is formed on the surface of the alloy particles. Thus activation of the alloy is made very difficult. In the final catalyst, this layer represents a diffusion barrier for reactant molecules, which results in a further loss in activity.
In addition, it is expected of modern catalyst systems that they should be easy to reclaim for re-use, in order to protect the environment. Processing of ceramically bonded metal fixed-bed catalysts however is difficult due to the insoluble ceramic binder.
EP 0 648 534 A1 describes the preparation of an activated metal fixed-bed catalyst which is produced without xcex1-aluminum oxide as a binder. The catalyst is obtained by shaping a powder of at least one catalyst alloy with a powder of the pure catalyst metal, with the addition of shaping aids and pore producers, and then calcining at temperatures of less than 850xc2x0 C. During calcination the shaping aids and pore producers are burned away. The alloy powder and metal powder sinter together to provide mechanically stable and porous shaped articles. These shaped articles thus consist of particles of the catalyst alloy which are bonded by a powder of the pure catalyst metal. They do not contain any catalytically inactive ceramic binder. The shaped articles are activated in a surface layer by extracting the aluminum contained in the catalyst alloys with caustic soda solution.
Although the pure catalyst metal used as binder in this catalyst also makes a certain contribution to the catalytic activity of the catalyst, its contribution is negligible due to the low specific surface area of this material. Thus the catalytic activity of the catalyst, with respect to the total weight of catalyst, is lower than it would be if the catalyst metal were not used as a binder.
EP 0 648 534 A1 recommends using, as binder, a powder of the pure catalyst metal with a particle size which is less than the particle size of the alloy powder in order to increase the strength of the catalyst-shaped articles. This leads to relatively dense catalysts with small pore volumes. EP 0 648 534 A1 does not mention the bulk densities of the catalysts. Fixed-bed catalysts prepared by this procedure, however, have very high bulk densities of about 2 kg/l.
Thermoplastic materials for preparing metallic shaped articles are disclosed in DE 40 07 345 A1. The materials contain A) a sinterable powdered metal or a powdered metal alloy, B) a mixture of B1) a polyoxymethylene homopolymer or copolymer and B2) a polymer, homogeneously dissolved in B1) or dispersed in B1) with an average particle size of less than 1 xcexcm, as binder and a dispersing aid. These materials may be shaped to give shaped articles. In order to remove the binder, the freshly prepared shaped articles obtained after shaping are treated under a gaseous acid-containing atmosphere. Treatment is performed until at least 80% of the polyoxymethylene fraction has been removed. Then the product obtained in this way is heated to 250 to 500xc2x0 C. in order to completely remove the remainder of the binder which is still present. The binder-free product can be converted into a metallic shaped article by sintering, the final product being free of cracks and pores even when the walls are thick.
An object of the present invention therefore is to provide a shaped, metal fixed-bed catalyst which has a substantially lower bulk density than comparable catalysts known from the prior art for the same or better hydrogenation activity.
The above and other objects of the present invention are achieved by a shaped metal fixed-bed catalyst which contains at least one catalyst alloy consisting of a catalyst metal and an extractable alloying component and optionally promoters. The catalyst is characterized in that it consists exclusively of the catalyst alloy(s), has a total pore volume of 0.1 to 0.6 ml/g and is activated in a surface layer with a thickness of 0.1 to 2.0 mm starting from the surface by complete or partial extraction of the extractable alloying component.
Thus the catalyst of the invention does not contain, in comparison to catalysts according to EP 0 648 534 A1, any less-active pure catalyst metal as a binder. In addition it has a higher pore volume than the known catalyst when using the same alloy powder, which means that a thicker activated outer layer is produced when using the same activation conditions. These differences result in a higher specific activity of the catalyst according to the invention both with respect to its weight and also to its bulk density.
Nickel, cobalt, copper or iron are preferably used as catalyst metals and aluminum, zinc or silicon are used as extractable alloying components. The ratio by weight of catalyst metal to extractable alloying component in the catalyst alloy is, as is conventional with Raney alloys, in the range from 30:70 to 70:30.